Typically, in gas turbine engines the impact or fan track liners have been bonded on to the fan case of the engine. In the past, this has been done in a large oven using a hot curing adhesive. Advantageously, bonding is a simple solution requiring minimal part count, minimal additional features on the case and is a relatively well understood process.
However, removal of the bonded liners from the case is not trivial. Removal of the liners may be necessary for repair/replacement reasons. In the past it has been proposed that sections of the liners can be cut and chiseled out to expose the bare case surface. The fan case surface can then be prepared using e.g. abrasive blasting so that a replacement liner section can be bonded to the exposed surface, e.g. using a cold curing adhesive, to fill the void.
However, such repair procedures are both difficult to perform and to control accurately. For example, the length of time it takes to perform the procedure, e.g. 30-40 hours, often means that the engine is removed so that the repair is performed off wing. This is a significant disruption if unscheduled.
There is also a chance of accidental tool damage to the case. This is a potentially more significant issue for a composite fan case.
Bolted liners have been proposed as an alternative solution. Bolted liners are relatively easy to replace, and therefore the processing time is less disruptive. However, using bolted liners introduces extra complexity and parts, e.g. nuts, bolts etc., and typically requires the case to be formed to include additional features to cooperate with the bolt-on liners to allow them to be mounted thereon. For example, mounting elements e.g. internal flanges are often required, and these structures can be difficult to incorporate into a composite fan case, for example.